In some forms of footwear the outer sole and the heel, which together provide the surfaces that contact the ground, are formed together as a unitary member, while in others they are formed separately and subsequently attached to one another and to the remainder of the boot or shoe. Such a unitary member will be referred to herein as a sole having a sole part or portion and a heel part or portion.
Many proposals have been made hitherto to produce footwear in which the resilience of the soles, particularly in the heel part thereof, is selected in accordance with the type of person and use for which the footwear is intended. Such selection of the resilience may consist of the choice of a particular degree for the entire sole including the heel part, and/or the choice of particular degrees of resilience for different specific parts of the sole. The choice for the heel part is of particular importance, since the major portions of the forces resulting from impacts with the ground are through that part.
For example, U.S. Pat. No. 4,043,058 to Hollister et al discloses an athletic training shoe employing a foam core sole layer and an apertured sole layer, the latter being sandwiched between a harder outer sole layer and the shoe upper. The apertured sole layer has a plurality of strategically placed vertically-extending apertures in order to provide lightweight cushioning at these locations, and preferably has a heel-lift layer positioned below an intermediate sole layer having a soft resilient foam core surrounded by a harder border portion.
U.S. Pat. No. 4,245,406 to Landsay et al discloses an athletic shoe in which an upper and a preformed rubber outsole are joined by means of a molding operation via a polyurethane injection-molded midsole, the midsole having a honeycomb like structure in the region behind the metatarsal area, the spaces in the honeycomb being filled during the molding with foamed polyurethane.
U.S. Pat. No. 4,399,620 to Herbert Funck describes a shoe with a unitary sole which comprises a lower layer of a suitable plastics or rubber material, and an upper layer of light-weight foam material, the thicknesses of the layers varying along the length of the sole and from side to side to provide a desired pattern of resilience.
U.S. Pat. No. 4,581,187 to Sullivan et al describes the production of a shoe innersole comprising a heel and arch section of molded polyurethane foam directly bonded in the molding process to a fullsole material which is either foam or a solid flexible sheet material.
There have been a number of prior proposals for footwear in which hollow air-receiving chambers formed in the sole and in the side walls can be pressurized as required via connecting passages and air valves to change the resilience of the sole and/or to press the side walls against the ankle for increased support. There have also been a number of proposals for footwear in which hollow chambers in the heel portion comprise bellows pumps that are actuated by the heel striking the road surface and pressurize the shoe as described above, and/or provide ventilating air to the shoe interior. Examples of such proposals are in U.S. Pat. Nos. 4,361,969; 4,744,157; 4,991,317; 4,995,173; 4,999,932; 5,025,575 and 5,117,566, and in PCT application PCT/AU92/00554 of J. M. O'Dwyer, published 29 Apr. 1993. All of the foregoing proposals require a multistep operation for the manufacture of the sole, while a further step or steps are required for its incorporation into a shoe by cementing in place, or by molding to the other elements of the shoe.
U.S. Pat. No. 4,435,523 of Ponzielli is concerned with a method of manufacturing articles of foamed thermoplastic material by injection molding, particularly wedge heels for women's shoes. Refrigerated compressed gas at high pressure is introduced into the mass of molten foaming thermoplastic material in order to cool it quickly and thereby reduce the molding cycle time. The specification states that as an incidental result of the method a sort of rough cavity or cavities are formed within the article as the compressed air presses the material against the outer walls of the mold; such voids are not sought and their shape, location, and existence are purely incidental in nature. Because of the random and unpredictable nature and location of the voids, and to ensure that they do not destroy the load bearing capability of the heel, Ponzielli provides vertical cores within the mold about which strengthening columns of tightly structured material are formed.
U.S. Pat. No. 5,204,051 of Jaroschek describes processes for the injection molding of fluid-filled plastic bodies in which a pressurized plastic melt is first injected by a nozzle into the mold cavity of a two or multi-part mold until the mold cavity is completely filled. In order to economize in plastic material and reduce the weight of the product, after cooling until the melt on the walls has set, a pressurized fluid is injected into the still molten interior to expel some of the material into at least one side cavity arranged outside the mold cavity, thereby facilitating the molding of complicated geometric shapes.